1. Field of the Invention
This invention relates to low-noise, crossed-field devices such as microwave magnetrons, microwave ovens utilizing same, crossed-field amplifiers and methods of converting noisy magnetrons to low-noise magnetrons.
2. Background Art
The noise generation mechanisms of linear electron beam devices are well known. Generally, fluctuations of cathode electron emission excite space charge waves, which propagate along the electron beam. Calculations and computations of noise figures in linear devices agree with experiments. Methods of noise suppression in linear tubes are at a very advanced stage. On the other hand, noise generation mechanisms in cross-field devices are not presently understood and predictive computational calculations do not exist. Methods of noise suppression in crossed-field devices have not previously been practically realized.
Existing magnetrons and crossed-field amplifiers use an azimuthally-symmetric, axial magnetic field, shown in FIGS. 1a and 1b (exterior dashed line in FIG. 1b). In a standard microwave oven magnetron such as the magnetron, generally indicated at 70, of FIG. 7, permanent magnets 72 generate about 1 kGauss on the face, resulting in about 1.7 kGauss on-axis, at the midpoint between the two magnets 72. The magnetron 70 also typically includes a microwave output post 73, a magnetic metal yoke 74, cooling fins 75, a vacuum envelope 76 which contains cavities, a metal box containing chokes 77 and electrical cathode/filament connections 78. Such standard noisy magnetrons generate a copious amount of microwave noise near the carrier and more widely-spaced sidebands, as shown in one of the data plots of FIG. 5.
As described by J. M. Osepchuk in the 1995 article entitled “The Cooker Magnetron as a Standard in Crossed-Field Research,” PROCEEDINGS OF THE FIRST INTERNATIONAL WORKSHOP ON CROSSED-FIELD DEVICES, Ann Arbor, Mich., Aug. 15-16, 1995, University of Michigan, “The existence of magnetron noise is assuming a very practical aspect. There are over 200 million microwave ovens in the world operating at 2.45 GHz. There also are plans for a wide variety of new ‘wireles’ services to operate with frequency allocations ranging from 1.5 GHz to 3.0 GHz and possibly even higher, especially at 5.8 GHz. There are some serious questions about the potential that some of these systems will encounter unacceptable interference from microwave ovens—i.e., the sideband noise. Thus the characteristics of microwave oven noise are being studied extensively and there are plans for interim and final (tighter) specifications to limit such noise through regulations originating in current activities of the CISPR community within the IEC (International Electrotechnical Commission). Because the noise is predominantly at low anode currents most of the time, microwave oven noise shows up as sub-millisecond pulses of noise. Some experts believe modern digital and spread-spectrum communication techniques can live with this. On the other hand, if discrete spurious signals show up especially at close to peak current, the RFI might not be tolerable. The magnitude of the peak noise or spurious in the worst cases is of the order of 100 dB above a pW as measured in a 1 MHz bandwidth or even higher (or similar numbers in units of μV/m as measured at 3 meters from the oven). At present some authorities are investigating peak limits near such levels along with limits 30 to 40 dB lower when using narrow video bandwidths (e.g. 100 Hz) to yield ‘average’ measures of the noise.”
As further described in the above-noted article, “Cooker magnetron noise, therefore, will attract regulatory pressure in the future at the same time that others, i.e., the DOE in the U.S., are pressuring for higher oven efficiency which is, in principle, associated with higher noise. At the same time there are other magnetron-driven ISM devices that may amplify the concern about noise, e.g., the microwave ‘sulfur’ lamps, that are very efficient light sources that some day may operate for many hours per night illuminating large areas in buildings and parking lots, etc. One can presume that users of magnetrons may be forced to find ways of reducing such noise. Otherwise, competing devices might for the first time in history pose a threat to the magnetron as the power source of choice for ovens and other power applications. In the past year there was the preliminary report of an efficient (67%), low voltage (600 Volts) multi-beam klystron suitable for microwave oven use. Its developers estimate that in three years problems of cost, size and weight might be resolved. The klystron poses no noise problems and has other advantages. One can expect controversial discussions of competing power sources at meetings such as those of IMPI (the International Microwave Power Institute).”
Since the above-noted article was written, several communications systems have developed in the unlicensed, 2.4 GHz radio spectrum:                1) cordless telephones operating at 2.4 GHz;        2) Bluetooth, a wireless communication system used for computers, which operates with a spread spectrum, frequency-hopping, full-duplex signal; and        3) IEEE 802.11 b and 802.11 g, a Complementary Code Keying-Orthogonal Frequency Division Multiplexing system used for computer Local Area Networks (LANs), operating in the frequency range from 2.4 GHz to 2.4835 GHz.Since these communication systems occupy the same region of the spectrum utilized by microwave ovens, there exists significant potential for interference from noisy magnetrons.        
U.S. Pat. No. 4,465,953 issued to Bekefi uses a magnetic configuration which modulates the radial magnetic field by an azimuthally, spatially-periodic array of magnets in a magnetron to generate free electron laser radiation.
U.S. Pat. No. 3,932,820 issued to Damon et al. discloses how the noise in a crossed-field amplifier output is reduced by providing a non-uniform magnetic field across the surface of a cathode. A curved magnetic field may be provided across the cathode or by providing a concave shaped cathode. Additionally, the cathode may be tilted with respect to the crossed magnetic field.
U.S. Pat. No. 4,709,129 issued to Osepchuk discloses a typical microwave power source for a microwave oven in which a microwave magnetron is supplied simultaneously with filament heater power and with anode voltage through an inductive reactance power supply.
U.S. Pat. No. 6,437,510 issued to Thomas et al. discloses a crossed-field amplifier or magnetron which has a cathode body portion and an anode which cooperates with a crossed magnetic field to maintain emitted electrons on cycloidal paths and amplify an input signal or develop a microwave or millimeter wave output signal in an interaction space.
U.S. Pat. No. 4,310,786 issued to Kumpfer discloses a magnetron electron discharge device preferably for use in microwave heating or cooking apparatus which has a cylindrical resonant anode structure surrounding a concentric electron emitting filament.